The presence of large deposits of oil shale in the high plateau, semi-arid region of the western United States has given rise to extensive efforts to develop methods for recovering shale oil from kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is, in fact, a misnomer; it is neither shale nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the ground surface or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, such as U.S. Pat. Nos. 3,661,423, 4,043,597, 4,043,598, and 4,192,554 which are incorporated herein by this reference.
The above-mentioned patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale. For example, it is described that such formation is explosively expanded to form a stationary fragmented permeable mass of formation particles, i.e., a rubble bed containing oil shale within the formation, referred to herein as an in situ oil shale retort, or merely as a retort. Retorting gases are passed through the fragmented mass to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale. One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishing a combustion zone or front in the retort and introducing an oxygen-supplying retort inlet mixture into the retort to advance the combustion zone downwardly through the fragmented mass. In the combustion zone, oxygen from the retort inlet mixture is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the fragmented mass, the combustion zone is advanced downwardly through the fragmented mass in the retort.
The combustion gas and the portion of the retort inlet mixture that does not take part in the combustion process pass through the fragmented mass on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbons, and a residual carbonaceous material.
The liquid products and the gaseous products are cooled by the cooler oil shale fragments in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, collect at the bottom of the retort and are withdrawn. An off-gas is also withdrawn from the bottom of the retort. Such off-gas can include carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, hydrocarbon aerosols, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process.
The retort off-gas can be withdrawn from the bottom of the retort through a product withdrawal drift that is in fluid communication with the fragmented mass in the retort. Such a drift, for example, can extend below the bottom of the retort and be in communication with the fragmented mass through one or more vertical raises or drawports. Alternatively, such a drift can extend laterally from the side of the retort at its bottom.
The off-gas produced during retorting can contain carbon monoxide and sulfur compounds such as hydrogen sulfide, both of which are toxic. It is therefore desirable to seal the withdrawal drift so that workers in adjacent underground workings are isolated from the off-gas produced in the fragmented mass during retorting operations. One such gas seal is disclosed in U.S. Pat. No. 4,294,563, which is incorporated herein by this reference.
During the earlier stages of the retorting operation, the retort off-gas is relatively cool, for example less than about 160.degree. F., since it flows through a bed of oil shale particles below the retorting zone that has not yet been heated. As retorting progresses and the combustion zone approaches the bottom of the retort, the off-gas temperature rises.
The oil yield efficiency for an in situ oil shale retort is a function of both the sweep efficiency of the retorting zone, i.e., the percentage of the fragmented mass through which the retorting zone has passed at the conclusion of the retorting process, and the retorting efficiency in that portion of the fragmented mass that was swept by the retorting zone. The retorting efficiency is largely dependent on the oil shale grade and particle size distribution of the fragmented mass. Sweep efficiency, on the other hand, is dependent on the permeability distribution in the fragmented mass and the design of the retort.
When the permeability distribution in the rubble is uneven, those portions of the combustion and retorting fronts that travel through regions of higher permeability can reach the bottom of the retort before portions that travel through regions of lower permeability. When this occurs, the temperature of the off-gas can start to increase while a large fraction of the rubble remains unretorted. In a large commercial-sized retort with a plurality of spaced apart drawports, even when the permeability in the rubble bed is uniform, the combustion and retorting fronts can be distorted. For example, as the combustion and retorting fronts approach the bottom of such a commercial-sized retort, those portions of the combustion and retorting fronts in the regions of the drawports can advance more rapidly than other portions. This effect plus the usual temperature gradient in the retorting zone, can result in a significant rise in the off-gas temperature before the entire fragmented mass is swept by the retorting zone. Thus, in either case, i.e., when the permeability of the fragmented mass is uneven or when a commercial-sized retort with a plurality of drawports is used, the temperature rise of the off-gas can begin before the entire fragmented mass has been swept.
The gas seal provided in the withdrawal drift can be a steel bulkhead which is cemented or grouted around its perimeter into a slot in the drift walls. As the temperature of the off-gas rises it causes increased thermal expansion and buckling of the steel bulkhead which in turn can result in damage to the grout seal and to the drift wall itself. This can result in loss of the integrity of the gas seal and failure of the bulkhead.
It has been found that the retort off-gas temperature can increase to above the maximum temperature for bulkhead safety well before the combustion and retorting fronts passing downwardly through the retort have swept the entire fragmented mass. Since retorting must be stopped for reasons of safety when the off-gas temperature in the withdrawal drift in the region of the bulkhead increases above the bulkhead safety temperature, the unswept portion of the fragmented mass in the retort is not retorted. This results in reduced product yields.
Additionally, when one portion of the combustion front reaches the bottom of the retort before other portions, combustion no longer occurs in that portion of the fragmented mass at the retort's bottom through which the advanced portion of the combustion front has passed. This leaves a pathway through the entire fragmented mass from its top to its bottom through which a portion of the retort inlet mixture can pass without the oxygen contained therein being consumed. This is called "oxygen breakthrough" and can result in the oxygen concentration in the off-gas increasing to unsafe levels before the entire fragmented mass is swept by the retorting zone. When this occurs retorting must be stopped and product yields are less than desired.
It is therefore desired to provide a retorting process that promotes lower off-gas temperatures in the vicinity of the gas sealing bulkhead and eliminates problems associated with oxygen breakthrough.